The road, rail or sea tankers used to transport crude oil normally need to be cleaned after each journey in order to get rid of the oily sludge left as a residue after removal of the main bulk of the contents. In the past tankers have been cleaned with water and a surfactant (or surfactant composition), the resulting emulsion of the sludge in water being thrown away--down the drain into the sewage system, or over the side into the sea, for example. However, such practice is frowned upon today not only because of the possible pollution resulting therefrom but also because quite often the sludge itself is potentially of not insignificant value.